We all know about Class A and Class A/B amplifiers. But what about Class D? Just about everyone calls them "Digital". But, did you know they are not really digital?

It is not necessarily a big deal to call Class D amplifiers "Digital". It is convenient to remember them that way, because of the "D". But the D was assigned simply because it was the next letter in the alphabet that was available, rather than because the design was digital.

Class D actually is a "Switching" circuit. What this means is that the rail voltage is turned completely on or completely off very quickly, and these tiny "jolts" create the waveform.

For CD players, which use PCM (Pulse Code Modulation), the circuit is indeed digital. A 16 or 24 bit number codes for a specific voltage on the waveform. But for switching amplifiers, PWM (Pulse Width Modulation) is used, which is analog. The output voltage is turned on at full value for varying lengths of time, at high frequency. The height of the waveform depends on how long the voltage is left on (the "Width").

Class D amplifiers have been around quite a while actually. The problem was that they didn't sound very good (with one or two exceptions) when used full range. Up until recently, they were used primarily as subwoofer amplifiers, where they perform beautifully, because they operate only in the low frequency range for that purpose.

Class D has begun to emerge as useful in full range products only in the last couple of years because the technology to switch at very high frequency - necessary for a full range switching amplifier - is now easier (and less expensive) to accomplish. In the next few years, we will see more and more Class D products for the home and for cars as well.

Along with several companies out there, Rotel has now entered the switching power amplifier arena with the RB-1092. It is a relatively small package (22 pounds) which produces 500 watts RMS into 8 ohms for each of two channels. This is a huge amount of power for an amplifier that you can basically lift with one hand.

The front panel is a nice simple design with a silver center piece flanked by small black heat sinks. There is a single on/off button with a blue LED above it and protection indicator lights. The back of the amplifier has a 12 volt trigger (in and out), and unbalanced RCA input connectors.

There are two sets of binding posts for each channel allowing for easy bi-wiring (if that's your thing). The binding posts are high quality and easy to tighten, allowing for bare wire, spades, or banana plugs. There are also Speakon® speaker connectors if you have the proper cables.

Setup

The setup I used for critical listening of the RB-1092 included a Holman preamplifier, Onix XCD-88 CD player, and a pair of B&W 804 speakers. The Rotel RB-1092 class D amplifier took the place normally reserved for a Luxman M-113 stereo amplifier. It should be noted that the Luxman, while rated at one tenth the power output of the Rotel, takes up twice the room that the Rotel does.

For this amp review I choose to use Pork Soda by Primus, Come Away with Me by Norah Jones, and Mule Variations by Tom Waits.

Here is a neat trick. Go to your local hi-fi shop, ask to listen to one of their more powerful mid-priced amplifiers hooked up to some juicy higher-end speakers, and drop in anything by Primus. Within moments, you will have the listening room to yourself.

If you can look past the unorthodox sound, you will find music that is an absolute delight to listen to. The band Primus is truly an oddity - their albums have comparatively good sound quality (considering the content), they are astonishingly tight musicians, and the songs are always interesting.

And they have a way of exploiting their instruments to deliver a sound that is far from ordinary. Really, really far from mundane.

This entire Pork Soda album has solid mid-bass coming from both Les Claypool's (lead) bass guitar and Tim Alexander's massive drum kit. The double kick drum on DMV played through the Rotel RB-1092 was solid and full. Larry Lalonde's guitar had its own layer, never stepping over the two other instruments. The man knows how to bend a note, and I could hear it right where it should be.

The bass lines had noticeably more impact than with my Luxman amp. Headroom has its benefits, and they were apparent with this amplifier. The biggest difference between the RB-1092 and my Luxman is that the layers are all distinct and clearly presented with the Rotel. At times they tend to meld and blur when played through the Luxman.

"Nature Boy" starts with a kick drum and high hat. This provides the beat for the first minute, then Claypool jumps all over it with a twisted bass line. The entire time Tim's beat still stays on his own level, and when he adds more drums to the mix, you hear him hit each one from the left side of his drum kit over to the right side. This is a feat I have not heard with any amplifier/speaker combination (a review of the B&W 804s is forthcoming).

Larry's surf-metal guitar wails almost knock everyone over. The whole time the original beat laid down by Tim is still audible. The Rotel RB-1092 easily handled the raw energy and detail of this three-piece band without compromising any of the layers. Sure there maybe only three layers in all; if you've ever listened to this disc you will know that is more than enough. The bass produced by this amplifier was clearly better than my Luxman. The reason for this is that Class D is very efficient, so nearly 100% of the available juice from the power supply is for music, not heat production. Deep bass needs plenty of current from the power supply.

A completely different musical experience, Norah Jones' Come Away with Me was next in queue. I was able to hear the layers of music clearer than with either my Marantz receiver or Luxman stereo amplifier. It felt as each note had just a little longer decay, as if the dynamic range were opened up and the quieter layers became more audible.

The disc was a whole new experience with 500 watts per side in reserve. Some amplifiers sound great at one end of the volume scale or the other. With this disc, the Rotel RB-1092 was most pleasant from mid to loud volumes, and increasing the volume did not cause any loss of detail or clarity. In fact, the sound opened up nicely. As with the Primus disc, the bass was ample and full, and this was with the B&Ws running without a subwoofer.

Next up I threw on Tom Waits' Mule Variations. This disc has more instruments and creative recording than most bands ever use in their entire catalog. Tom's voice is an instrument unto itself, and the Rotel clearly defined it with only slight midrange bloom noticeable on some tracks. A standout song on this disc is "Chocolate Jesus". The recording was made outdoors and has some impromptu backup vocals by the local rooster.

With the Luxman, the rooster appeared to be in the recording with the rest of the music, nothing special. Once the Rotel amplifier was introduced into the mix, the layers of the music opened up, and the rooster was coming from the far back corner behind the speakers. The extra oomph in the low end provided by the amplifier did have the negative effect of overemphasizing the low frequency noise present in this song. However, the rest of the disc benefited greatly from the solid bass provided by the Rotel.